The invention relates to a rheostatic device in a switching arrangement, particularly for controlling the speed of electric motors or the like, including a switch housing and a circuit board disposed in the switch housing for receiving electronic components. A resistance track is provided on the circuit board for controlling resistance. A potentiometer slider slides on the resistance track using an actuating member.
A continuous control of the speed of electric tools such as, for example, drilling machines, is effected by means of electronic switches or electronics modules. The actuation of a push button or of an adjusting wheel acts on a controlling element which results in a change in an electric measured quantity. In general, a potentiometer is used as the controlling element which is constructed as a part of the switch or of the electronics module. In this arrangement, the push-button or the adjusting wheel is provided with a potentiometer slider which slides on an associated resistance track. This is printed onto a large-area carrier board or circuit board, the circuit board usually being used at the same time as carrier for circuit tracks and other electric constructional elements and other components.
Special technologies are applied in producing circuit boards. Due to the different requirements, various basic materials--so-called hard papers--are used. These are, for example, copper-clad hard paper on phenolic resin base on epoxy resin base or copper-clad laminated pressed material of epoxy resin glass silk fabric, that is to say glass-fibre reinforced. The basic material can be provided on one side or on both sides with a copper foil, the copper foil being protected with an etching varnish in the screen printing method or with a photoresist in the photographic method at the points at which the conductor runs and soldering eyes are to be produced. The exposed copper is then etched away.
If such a circuit board is to be additionally provided with a potentiometer track or resistance track, this is also applied, for example in a screen printing method, in the form of a paste onto the base material and fired. During this processing operation, however, the copper conductor tracks already produced are influenced by oxidation so that post-treatment is again required. In particular, negative chemical and thermal interactions occur between the etched circuit board and the applied resistance track during the firing processes. The resistance value of the applied resistance track is then subject to a very large manufacturing tolerance which is caused by material and production processes. This tolerance can be up to .+-.20% of the nominal resistance value. The additional application of a carbon layer track as a resistance track on a circuit board also represents an additional production risk in the production of the switch or electronics module. Furthermore, the resistance track applied in known technology to the circuit board is susceptible to environmental influences such as, for example, humidity, temperature and furthermore to out-gassing of the carrier material during the firing process of the conductor track which results in a drift characteristic of the electrical value both during the production and during the utilization.
The production of a resistance track on the existing circuit board of a switch or of an electronics module in conjunction with the utilization of firing pastes (R paste) also has the disadvantage of increased wear during operation if only lower firing temperatures can be used due to the carrier materials used.
The application of resistance tracks to circuit boards to be equipped conventionally for producing a switch or an electronics module does not allow optimum handling of the circuit board itself. In particular, there is no control characteristic occurring analogously to the remaining electronic components arranged on the circuit board which, however, would be required for automatic, modern production of fully electronic units.
The so-called SMD (surface mounted device) technology was introduced into industrial products some years ago. In particular, this technology is used for thin film and thick film circuits but also for miniaturizing board structures. The basis of this new technology is formed by miniaturized components (SMDs=surface mounted devices) which are directly mounted on the surface of circuit boards or ceramic substrates. In particular, SMD chips are also suitable for processing by means of automatic insertion equipment and for all soldering methods, such as, for example, dip soldering or reflow soldering. After being fixed to the circuit board by means of an adhesive drop they can therefore be moved together with this board through the soldering bath. SMDs can be used on one side of the circuit board, on both sides or in mixed equipment with conventionally wired components. The mixed equipment is suitable when the intended circuit cannot be implemented by means of SMDs alone.
The main advantages of SMD technology lie in the miniaturization, in advantageous RF characteristics, in a very high quality standard and in a cost reduction both in the case of the components and in the case of the circuit bard.
So-called SMD chip resistors have also become known as components in SMD technology. These, like the other SMD components, have a uniform constructional size of 3.2 mm length, 1.6 mm width and 0.6 mm thickness. A ceramic material of high purity (aluminium oxide, silicon) is used as substrate for applying the resistance layer as is generally done in the screen printing method. The composition of the layering of the resistance track is selected in such a manner that the desired resistance is approximately achieved. The rated resistance can then be fixed by so-called laser trimming. To protect the resistance area, the component is covered with a glaze. At the ends of the component, end contacts of metal are applied which can be constructed of several layers. To achieve a reliable connection between end contacts the and resistance layer, a base minaturization is provided.
In addition to such SMD chip resistors, so-called SMD ceramic multi-layer chip capacitors have also become known as components which are constructed in accordance with known methods. Furthermore, tantalum chip capacitors or electrolytic aluminium chip capacitors are known. Finally, transistors and diodes which are uniformly adapted to the dimensions exist as SMD components.
From DE 37 41 175 A1, a variable resistor has become known which can be handled as an insulated component also in SMD technology, that is to say, could be soldered onto a board as a separate component. This component consists of a carrier layer with an applied resistance track and inserted potentiometer slider. Such a variable resistor could also be used on a board in SMD technology, particularly as a trimming potentiometer. It has the advantage that it can be produced as a separate component independently of the beard production.